"Plugin could not be activated because it triggered a fatal error." in WordPress

The recommended way to write that function would be:

function ce3_cleanup() { global $wpdb; $wpdb->
query( "update $wpdb->
postmeta set meta_key=replace(meta_key,'cb2_customHeader','_cb2_customHeader') where meta_key like 'cb2_customHeader'" ); $wpdb->
query( "update $wpdb->
postmeta set meta_key=replace(meta_key,'cb2_customTitle','_cb2_customTitle') where meta_key like 'cb2_customTitle'" );
}
register_activation_hook( __FILE__, 'ce3_cleanup' );